MAINTAINERS: correct git location for block layer tree

As part of a recent move go exclusively listing git.kernel.org trees
for the block and io_uring development, the "BLOCK LAYER" entry wasn't
updated as it already used git.kernel.org. However, outside of just
moving from git.kernel.dk to git.kernel.org, the "block" part of the
trees was also dropped, as the tree serves both block and io_uring
development trees.

Fix up the "BLOCK LAYER" entry so they all use the same tree.

Reported-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>